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Abstract:

Provided are an alumina sintered compact containing a titanium compound
and an iron compound, wherein the total amount of the
TiO2-equivalent content of the titanium compound, the
Fe2O3-equivalent content of the iron compound and the alumina
content is at least 98% by mass, the total amount of the
TiO2-equivalent content of the titanium compound and the
Fe2O3-equivalent content of the iron compound is from 5 to 13%
by mass, and the ratio by mass of the TiO2-equivalent content of the
titanium compound to the Fe2O3-equivalent content of the iron
compound (TiO2/Fe2O3) is from 0.85/1.15 to 1.15/0.85; and
an abrasive grain and a grain stone using the alumina sintered compact.

Claims:

1. An alumina sintered compact containing a titanium compound and an iron
compound, wherein: the total amount of the TiO2-equivalent content
of the titanium compound, the Fe2O3-equivalent content of the
iron compound and the alumina content is at least 98% by mass, the total
amount of the TiO2-equivalent content of the titanium compound and
the Fe2O3-equivalent content of the iron compound is from 5 to
13% by mass, and the ratio by mass of the TiO2-equivalent content of
the titanium compound to the Fe2O3-equivalent content of the
iron compound (TiO2/Fe2O3) is from 0.85/1.15 to 1.15/0.85.

2. The alumina sintered compact according to claim 1, wherein the total
amount of the TiO2-equivalent content of the titanium compound and
the Fe2O3-equivalent content of the iron compound is from 7 to
10% by mass.

4. A grind stone having a layer of the abrasive grains of the claim 3 as
the working face thereof.

Description:

TECHNICAL FIELD

[0001] The present invention relates to an alumina sintered compact, an
abrasive grain using the alumina sintered compact, and a grind stone
using the abrasive grain.

BACKGROUND ART

[0002] An alumina sintered compact is used in various industrial fields,
making full use of the excellent characteristics thereof of high
hardness, high strength, high heat resistance, high abrasion resistance,
high chemical resistance, etc. In particular, it is used as a starting
material (abrasive grain) of heavy grinding stones in steel industry.

[0003] Special alloys are being much used as a material for parts
constituting transportation equipment centered on automobiles or
industrial machinery. These special alloys are hard as compared with
ordinary SUS304 and others, and heavy grinding stones heretofore unknown
and having a high "grinding ratio" are desired by the market. In this,
"grinding ratio" is an index of indicating the performance of grind
stones and is represented by the following formula:

[0004] In general, it is considered that a grind stone requiring a smaller
amount thereof to grind a larger amount of a work material could have
better performance; however, the grinding ratio of a grind stone is
influenced by the "hardness" and the "fracture toughness" of the abrasive
grains used for the grind stone. It is considered that there would be the
following relationships between "the grinding ratio and the hardness",
and "the grinding ratio and the fracture toughness".

(1) When the hardness of an abrasive grain is high, then the ground
amount increases and therefore the grinding ratio becomes high. (2) When
the fracture toughness is high, then the abrasion loss of the abrasive
grain reduces and therefore the grinding ratio becomes high.

[0005] In consideration of the above (1) and (2), the numerator part in
the formula of grinding ratio is influenced by the ground amount, and the
denominator part is influenced by the abrasion loss. For increasing the
grinding ratio of a grind stone, it is ideal that both the hardness and
the fracture toughness thereof are high.

[0007] Also proposed is a sintered material comprising aluminium material
as the main ingredient and TiO2 added thereto (for example, see
Patent Reference 6). Further, as an alumina sintered compact having high
hardness and high fracture toughness and excellent in abrasion
resistance, proposed is an alumina sintered compact in which a soluble
metal compound of Ti, Mg, Fe or the like is added to the alumina crystal
(for example, see Patent Reference 7).

CITATION LIST

Patent References

[0008] [Patent Reference 1] JP-B 39-4398

[0009] [Patent Reference 2]
JP-B 39-27612

[0010] [Patent Reference 3] JP-B 39-27614

[0011] [Patent
Reference 4] JP-B 39-16592

[0012] [Patent Reference 5] JP-B 52-14993

[0013] [Patent Reference 6] JP-A 3-97661

[0014] [Patent Reference 7] JP-A
11-157962

SUMMARY OF THE INVENTION

Problems that the Invention is to Solve

[0015] However, the abrasive grains in Patent References 1 to 5 all have
high hardness but low fracture toughness or have low hardness but high
fracture toughness, and these references do not concretely disclose
abrasive grains that have high hardness and high fracture toughness. In
Patent Reference 6, the hardness of the sintered material is evaluated,
but nothing relating to the fracture toughness thereof is taken into
consideration therein. Patent Reference 7 discloses, as the alumina
sintered compact therein, only a combination of Ti and Mg and a
combination of Fe and Mg, but does not concretely disclose any other
combination.

[0016] Given the situation as above, the present invention has been made
and its object is to provide an alumina sintered compact capable of
giving abrasive grains having high hardness and excellent in fracture
toughness, an abrasive grain using the alumina sintered compact, and a
grind stone using the abrasive grain.

Means for Solving the Problems

[0017] The present inventors have assiduously studied for the purpose of
attaining the above-mentioned object and, as a result, have specifically
noted, as the compound to be contained in the alumina sintered compact, a
titanium compound (especially titanium oxide) and an iron compound
(especially iron oxide), and have found that, when the total amount of
those compounds (the total amount of the content of those compounds as
their oxides) is controlled, then the properties of the alumina sintered
compact can be bettered. The present invention has been completed on the
basis of these findings.

[0018] Specifically, the present invention is as described below.

[1] An alumina sintered compact containing a titanium compound and an
iron compound, wherein the total amount of the TiO2-equivalent
content of the titanium compound, the Fe2O3-equivalent content
of the iron compound and the alumina content is at least 98% by mass, the
total amount of the TiO2-equivalent content of the titanium compound
and the Fe2O3-equivalent content of the iron compound is from 5
to 13% by mass, and the ratio by mass of the TiO2-equivalent content
of the titanium compound to the Fe2O3-equivalent content of the
iron compound (TiO2/Fe2O3) is from 0.85/1.15 to 1.15/0.85.
[2] The alumina sintered compact of the above [1], wherein the total
amount of the TiO2-equivalent content of the titanium compound and
the Fe2O3-equivalent content of the iron compound is from 7 to
10% by mass. [3] An abrasive grain comprising the alumina sintered
compact of the above [1] or [2]. [4] A grind stone having a layer of the
abrasive grains of the above [3] as the working face thereof.

Advantage of the Invention

[0019] According to the present invention, there are provided an alumina
sintered compact capable of giving abrasive grains having high hardness
and excellent in fracture toughness, an abrasive grain using the alumina
sintered compact, and a grind stone using the abrasive grain.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] FIG. 1 This is an action explanatory view of explaining the mode of
impact propagation to the alumina sintered compact of the present
invention.

[0021] FIG. 2 This shows SEM pictures of the alumina sintered compact of
the present invention before and after impact test thereof; (A) shows the
condition of the crystalline structure before impact test (processed by
thermal etching); (B) shows the condition of crack propagation after
impact test (not processed by thermal etching).

[0022] FIG. 3 This is an action explanatory view of explaining the mode of
impact propagation to a sintered compact of alumina alone.

[0023] FIG. 4 This shows SEM pictures of the sintered compact of alumina
alone before and after impact test thereof; (A) shows the condition of
the crystalline structure before impact test (processed by thermal
etching); (B) shows the condition of crack propagation after impact test
(not processed by thermal etching).

[0024] FIG. 5 This is an X-ray diffraction pattern showing the result of
compositional analysis (X-ray diffractiometry) of the alumina sintered
compact of Example 4.

MODE FOR CARRYING OUT THE INVENTION

[Alumina Sintered Compact]

[0025] The alumina sintered compact of the present invention contains a
titanium compound and an iron compound, wherein the total amount of the
three ingredients of the TiO2-equivalent content of the titanium
compound (hereinafter this may be referred to as "TiO2-equivalent
content"), the Fe2O3-equivalent content of the iron compound
(hereinafter this may be referred to as "Fe2O3-equivalent
content") and the alumina content is at least 98% by mass.

[0026] The total amount of the two ingredients of the TiO2-equivalent
content and the Fe2O3-equivalent content is from 5 to 13% by
mass, preferably from 7 to 10% by mass.

[0027] In the alumina sintered compact of the present invention, the ratio
by mass of the TiO2-equivalent content to the
Fe2O3-equivalent content (TiO2/Fe2O3) is from
0.85/1.15 to 1.15/0.85, in order that the sintered compact can have high
hardness and high fracture toughness.

[0028] Preferably, the above ratio (TiO2/Fe2O3) is from
0.90/1.10 to 1.10/0.90, more preferably from 0.95/1.05 to 1.05/0.95.

[0029] Regarding the relationship between the total amount of the two
ingredients of the TiO2-equivalent content and the
Fe2O3-equivalent content, and the hardness, when the total
amount is larger, then the hardness is lower; however, in case where the
total amount of the two ingredients falls within the range defined in the
present invention, then the mean Vickers hardness that is an index of
hardness is, for example, at least 16 GPa, therefore indicating the
presence of a practically excellent hardness.

[0030] On the other hand, the relationship between the total amount of the
two ingredients and the fracture toughness is not like the relationship
to the hardness as above; however, the present inventors have found that,
within a specific range of the total amount of the two ingredients, the
fracture toughness is extremely high. Specifically, when the total amount
of the two ingredients falls within the range defined in the present
invention, then the fracture toughness value may be, for example, at
least 3.0 MPam1/2.

[0031] Here the mechanism that could provide the above-mentioned effect is
described below.

[0032] First, in a sintered compact of alumina alone, the crack
propagation runs in the direction of the arrow Y along the grain boundary
of the alumina grains 12, as shown in FIG. 3. With that, depending on the
impact level, a linear crack may form along the grain boundary, as shown
in the SEM picture of FIG. 4(B). FIG. 4 shows SEM pictures of the
sintered compact of Comparative Example 1 to be mentioned below; and FIG.
4(A) shows the condition of the crystalline structure before given
impact, and FIG. 4(B) shows the condition of crack propagation after
given impact.

[0033] On the other hand, incorporation of a titanium compound and an iron
compound provides a crystal phase of a composite metal oxide having a
high fracture toughness value (for example, FeTiAlO5 grains 10) in
the grain boundary of the alumina grains 12, as shown in FIG. 1. The
FeTiAlO5 grains 10 thus exist in the grain boundary of the alumina
grains 12, and therefore, even though the crack formed by impact
application grows further, the crack could be deviated so as to go by a
roundabout route in the direction of the arrow X from the starting point
of the grain 10, and consequently, the impact force is not in one
direction but diffuses and is thereby relaxed. Accordingly, it is
considered that the fracture toughness value would be high as a whole.

[0034] This is known from the SEM pictures of FIG. 2 indicating the
results of impact test. Specifically, when impact is given in the
condition where the FeTiAlO5 grains exist in the grain boundary of
the alumina grains, as in the SEM picture of FIG. 2(A), then the crack
starting from the FeTiAlO5 grains may go around the grains, as in
FIG. 2(B).

[0035] FIG. 2 shows SEM pictures of the sintered compact of Example 3 to
be mentioned below, in which the gray part (tinted part) positioned in
the triple point existing in the grain boundary of the alumina grains
corresponds to the FeTiAlO5 grain.

[0036] As described in the production method for an alumina sintered
compact to be mentioned below, use of ilmenite (titanic iron:
FeTiO3) as the starting material containing Ti and Fe is preferred
from the viewpoint of the production cost.

[0037] In the alumina sintered compact of the present invention, a crystal
phase of a composite metal oxide with Ti, Fe and Al, concretely, the
FeTiAlO5 grain exists in the grain boundary of the main crystal
phase composed of a corundum crystal, as described above. The existence
of the FeTiAlO5 grains provides the alumina sintered compact that
gives abrasive grains having high hardness and excellent in fracture
toughness. In particular, owing to the effect of the FeTiAlO5 grains
having higher fracture toughness than a corundum phase, there can be
obtained an alumina sintered compact having high hardness and excellent
fracture toughness. The presence of the crystal phase comprising
FeTiAlO5 grains and the mean grain size thereof can be confirmed
according to the methods described in Examples to be given below.

[0038] The mean grain size of the crystal phase of the composite metal
oxide with Ti, Fe and Al (FeTiAlO5 grains) is preferably from 3.4 to
7.0 μm, more preferably from 3.7 to 6.5 μm, from the viewpoint of
increasing the fracture toughness. Falling within the range of from 3.4
to 7.0 μm, the grains could be more effective for preventing the
growth of the cracks formed by fracture. This is because, when the mean
grain size falls within the range, the effect of the FeTiAlO5 grains
for deviating the running route of cracks can be secured well.

[0039] Preferably, the alumina sintered compact of the present invention
contains a silicon compound and/or a calcium compound that are other
metal compounds than TiO2, Fe2O3 and Al2O3, in
order that the sintered compact could have higher fracture toughness.

[0040] Preferably, the total amount of the SiO2-equivalent content of
the silicon compound (hereinafter this may be referred to as
"SiO2-equivalent content") and the CaO-equivalent content of the
calcium compound (hereinafter this may be referred to as "CaO-equivalent
content") is at most 2% by mass, more preferably from 0.5 to 2% by mass.

[0041] The silicon compound and the calcium compound act as a grain
growing agent, and it is considered that the presence of at most 2% by
mass, as their oxides, of these compounds would unhomogenize the shape
and the size of the alumina corundum grains therefore causing deviation
of cracks. Specifically, it is considered that, owing to the existence of
a specific amount of a titanium compound and an iron compound and a
specific amount of a silicon compound and a calcium compound, the
respective effects could be combined therefore efficiently providing
deviation of cracks and attaining the effect of further increasing the
fracture toughness.

[0042] Here the alumina content, the TiO2-equivalent content, the
Fe2O3-equivalent content, the SiO2-equivalent content, the
CaO-equivalent content and the metal oxide-equivalent content of other
metal compounds are determined according to a fluorescent X-ray
elementary analysis method. Concretely, they are determined as follows.

[0043] First, for the measurement, a standard oxide sample of which the
elementary composition is known is analyzed in wet. With the thus-found,
wet analysis data taken as the standard values, calibration curves
necessary for measurement are formed. The quantitative analysis of the
samples is carried out on the basis of the thus-formed calibration
curves. As the measurement apparatus, usable is Panalytical's "PW2400
Model". For the measurement, preferably, the condition is such that the
tube is a rhodium tube is used and the characteristic X ray is a Kα
ray. Preferably, in the measurement, the tube voltage and the tube
current are varied for the individual elements. Examples of the
conditions of the tube voltage and the tube current are shown in Table 1
below.

[0044] In this specification, the entire amount to be the denominator in
determining the individual metal oxide-equivalent content is the total
amount of all the metal elements, as their oxides, contained in the
alumina sintered compact.

[0045] Next described is a method for producing the above-mentioned
alumina sintered compact of the present invention.

(Starting Material)

[0046] In the method for producing the alumina sintered compact of the
present invention, an alumina, a titanium compound and an iron compound
are used as the starting materials. If desired, a silicon compound and/or
a calcium compound may be further used. These may be in the form of a
composite oxide containing two or more of them.

[0047] Regarding the form of the starting material, there are mentioned a
powder, a metal powder, a slurry, an aqueous solution, etc. In the
present invention, preferably, the starting materials are in the form of
powder from the viewpoint of easiness in handling them in operation. In
case where powdery starting materials are used, the cumulative mass 50%
diameter (d50) of the alumina powder, the titanium compound powder,
the iron compound powder, the silicon compound powder and the calcium
compound powder is preferably at most 3 μm each, more preferably at
most 1 μm for obtaining a homogenous mixed powder.

[0048] Here, the cumulative mass 50% diameter (d50) of the powders
can be determined according to a laser diffraction method.

[0049] The alumina powder is the starting material for forming the main
crystal phase of a corundum crystal in the alumina sintered compact to be
obtained, and is therefore preferably a high-purity one, and for example,
preferred is use of alumina or the like formed according to a Bayer
process.

[0050] The titanium compound powder and the iron compound powder may be a
high-purity TiO2 powder and a high-purity Fe2O3 powder,
respectively, or may also be in the form of a composite oxide of all or
two of titanium, iron and alumina. The composite oxide includes ilmenite
(titanic iron: FeTiO3) powder, aluminium titanate powder,
FeTiAlO5 powder, etc. The ilmenite powder is more inexpensive than
high-purity TiO2 powder and high-purity Fe2O3 powder, and
therefore can lower the production cost of abrasive grains. Accordingly,
use of ilmenite powder is preferred.

[0051] Here ilmenite is also called titanic iron, and a
naturally-occurring iron and titanium oxide mineral, and its composition
is expressed as FeTiO3. The locality includes Australia, Norway,
Russian Ural region, India, Canada, America, Malaysia, etc., and the
chemical composition varies depending on the locality. There exist
derivatives of FeTiO3 in which Fe2+ is partly substituted with
Mg2+.

[0052] The chemical composition of the alumina ingredient of the
ingredients constituting ilmenite (from Queensland in Australia), and the
oxide-equivalent content of iron compound, titanium compound, silicon
compound and calcium compound are shown in Table 2 below.

[0053] In case where an ilmenite powder is used, the blend ratio by mass
of the ilmenite powder to the alumina powder (ilmenite powder/alumina
powder) is preferably from 0.05/0.95 to 0.16/0.84, more preferably from
0.08/0.92 to 0.12/0.88. When the blend ratio by mass is from 0.05/0.95 to
0.16/0.84, then the total amount of the two ingredients of the
TiO2-equivalent content and the Fe2O3-equivalent could be
from 5 to 13% by mass.

[0054] In case where a silicon compound and a calcium compound are used,
the SiO2-equivalent content of the silicon compound and the
CaO-equivalent content of the calcium compound are so controlled as to be
at most 2% by mass in total, preferably from 0.5 to 2% by mass. Using
them can further increase the fracture toughness value.

[0055] The silicon compound powder and the calcium compound powder may be
a high-purity SiO2 powder and a high-purity CaO powder, calcium
carbonate powder or the like, respectively; or may also be in the form of
a composite oxide of all or two of silica, calcium oxide and alumina. As
the composite oxide, there are mentioned powders of mullite, zeolite,
bentonite, gehlenite, anorthite, etc.

(Preparation of Mixture)

[0056] In the method for producing the alumina sintered compact of the
present invention, the method of preparing the starting material mixture
is not specifically defined. For example, the following method is
preferably employed here.

[0057] First, an alumina powder prepared according to a Bayer process and
an ilmenite powder (or TiO2 powder and Fe2O3 powder) each
in a predetermined amount are added to an aqueous medium containing
polyvinyl alcohol. Subsequently, for example, using an ultrasonic
disperser, a media-assisted disperser such as a planetary ball mill, a
ball mill, a sand mill or the like, or a medialess disperser such as
Altimizer (trade name), Nanomizer (trade name) or the like, a homogeneous
slurry is obtained. Next, the slurry is dried and then ground to prepare
a mixture (powder) having a cumulative mass 50% diameter (d50) of at
most 3 μm, preferably at most 1 μm.

(Sintering of Mixture)

[0058] A shaped compact of the starting material mixture prepared in the
manner as above is sintered to give the alumina sintered compact of the
present invention having a relative density of at least 95%, preferably
at least 97%. Having a relative density of at least 95%, reduction in the
hardness and the fracture toughness of the sintered compact to be caused
by the pores and voids in the sintered compact can be prevented. The
relative density can be computed by dividing the bulk density of the
sintered compact, as measured according to an Archimedian method, by the
true density thereof.

[0059] In sintering, the mixture is shaped to have a desired form
according to a known shaping method of, for example, mold pressing, cold
isostatic pressing, cast molding, injection molding, extrusion molding or
the like, and thereafter the shaped compact is sintered according to a
known sintering method, for example, according to various sintering
methods of a hot-pressing method, a normal pressure firing method, a
vapor pressure firing method, a microwave-heating firing method or the
like.

[0060] Thus obtained, the alumina sintered compact of the present
invention has high hardness and excellent fracture toughness, and is
favorable for, for example, grinding, cutting, polishing or the like
tools for grinding materials, cutting materials, polishing materials,
etc., and further for abrasive grains of heavy grinding stones in steel
industry.

[Abrasive Grain]

[0061] The abrasive grain of the present invention comprises the alumina
sintered compact of the present invention. The alumina sintered compact
of the present invention can be obtained through grinding treatment,
kneading treatment, shaping treatment, drying treatment and sintering
treatment to be attained sequentially.

[Grind Stone]

[0062] The grind stone of the present invention has a layer of the
abrasive grains of the present invention, as the working face thereof.

[0063] As the method of fixing the abrasive grains to the working face of
the grind stone of the present invention, there may be mentioned resin
bonding, vitrified bonding, metal bonding, electrodeposition, etc.

[0064] As the material of the core, there may be mentioned steel,
stainless alloys, aluminium alloys, etc.

[0065] Resin bonding provides sharp cutting, but is poor in durability.
Vitrified bonding provides sharp cutting and is good in abrasion
resistance, but gives internal stress to the abrasive grains, whereby the
abrasive grains may be often broken or cracked. Electrodeposition gives
broad latitude in shape and provides sharp cutting.

[0066] In view of the above, the fixation method may be selected in
accordance with the use of the grind stone.

[0067] Concretely, for example, in a case of a resin-bonded grind stone,
there may be employed a method comprising mixing a powder of a binder
such as a phenolic resin, a polyimide resin or the like and abrasive
grains, or coating abrasive grains with a binder, then filling them in a
mold followed by shaping it by pressing, or a method comprising mixing a
liquid binder such as an epoxy resin, an unsaturated polyester resin or
the like and abrasive grains, then casting them into a mold followed by
curing it, whereby there is obtained a grind stone of the present
invention that has a layer of abrasive grains fixed on the working face
thereof.

[0068] Not specifically defined, the shape of the grind stone of the
present invention may be suitably selected from a straight type, a cup
type or the like in accordance with the use of the grind stone.

EXAMPLES

[0069] Next, the present invention is described in more detail with
reference to Examples; however, the present invention is not whatsoever
limited by these Examples.

[0070] The properties in Examples were determined according to the methods
mentioned below.

[0073] As the apparatus, used was Akashi's Model "MVK-VL, Hardness
Tester". Regarding the measurement condition, the load was 0.98 N and the
indenter application time was 10 seconds. Under the condition, each
sample was analyzed on 15 points, and the found data were averaged to
give the mean Vickers hardness of the sample. Those having a mean Vickers
hardness of at least 16 GPa are free from problem in practical use.

(3) Mean Fracture Toughness Value of Alumina Sintered Compact:

[0074] As the apparatus, used was Matsuzawa Seiki's Model "DVK-1".
Regarding the measurement condition, the maximum load was 9.81 N, the
indenter application speed was 50 μm/sec, and the indenter application
time was 15 seconds. Under the condition, each sample was analyzed on 15
points, and the found data were averaged to give the mean fracture
toughness value of the sample. The computational formula is given below.
Those having a mean fracture toughness value of at least 3.0 MPam1/2
are free from problem in practical use.

[0076] As the apparatus, used was JEOL's Model "JSM-6510V" with which SEM
pictures were taken. On the SEM pictures, the mean grain size of each
crystal phase was measured. The mean grain size was measured as follows:
According to a diameter method, the maximum length in the same direction
of each grain (50 grains) was measured, and the found data were averaged
to give the mean grain size of the sample.

[0077] As the apparatus, used was Panalytical's Model "X' pert PRO". The
metal oxide crystal phase was analyzed for the composition thereof under
the condition that a CuKα ray was used as the characteristic X ray,
the tube voltage was 40 kV and the tube current was 40 mA.

(6) Relative Density:

[0078] The relative density was computed by dividing the bulk density of
the sintered compact, as measured according to an Archimedian method, by
the true density thereof.

[0079] In this, it was presumed that all the iron compound and the
titanium compound added could react to give FeTiAlO5. With that, the
true density of alumina was considered as 3.98 and the true density of
FeTiAlO5 was 4.28, and based on the proportion of the formed
FeTiAlO5 and the proportion of the remaining alumina, the true
density of the sample was computed.

[0080] As described above, the form of the starting materials for the
sintered compact includes powder, metal power, slurry, aqueous solution,
etc. In this Example, from the viewpoint of easy handlability thereof in
operation, it was considered that powdery starting materials would be
preferred, and therefore powdery starting materials were used. The
chemical composition of the alumina power, the ilmenite powder, the iron
oxide powder and the titanium oxide powder (as alumina content,
TiO2-equivalent content, Fe2O3-equivalent content,
SiO2-equivalent content, CaO-equivalent content) are shown in Tables
3 to 6 below.

[0082] The above ilmenite powder was from Australia, and was a product by
CRL (Consolidated Rutile Limited) in Australia. Before use herein, the
powder was ground to have a cumulative mass 50% diameter (d50) of
0.75 μm.

[0085] The above alumina powder having a cumulative mass 50% diameter
(d50) of 0.6 μm and the above ilmenite powder having a cumulative
mass 50% diameter (d50) of 0.75 μm were mixed in such a manner
that the TiO2 content and the Fe2O3 content in the alumina
sintered compact to be formed could be as in Table 7, thereby preparing
various mixtures.

[0086] 300 g of an aqueous solution containing 5% by mass of polyvinyl
alcohol and 600 g of pure water were added to each mixture, ground and
mixed in a ball mill (for 4 hours in Examples 1 to 5 and Comparative
Examples 1 to 6, but for 8 hours in Comparative Example 7), thereby
preparing various types of homogeneous slurries each having a mixture
concentration of about 25% by mass.

[0087] Next, each slurry was dried at 120° C. for 24 hours, and
then ground in a mortar to give a ground powder having a cumulative mass
50% diameter (d50) of at most 300 μm. Each ground powder was
molded in a mold under a pressure of 100 MPa, and then further processed
for hydrostatic pressurization under a pressure of 150 MPa to give
various types of molded compacts.

[0088] Subsequently, each molded compact was fired in an electric furnace
(air atmosphere) for 4 hours so as to have a relative density of at least
95%, thereby giving various alumina sintered compacts. These were tested
(evaluated) as above. The results are shown in Table 7 below.

[0089] FIG. 2 shows SEM pictures of the alumina sintered compact of
Example 3 before and after impact test; FIG. 4 shows SEM pictures of the
alumina sintered compact of Comparative Example 1 before and after impact
test. In these drawings, (A) shows the condition of the crystalline
structure before impact test, and (B) shows the condition of crack
propagation after impact test.

Examples 6 and 7 and Comparative Examples 8 and 9

[0090] The above alumina powder having a cumulative mass 50% diameter
(d50) of 0.6 μm, the above iron oxide powder having a cumulative
mass 50% diameter (d50) of 0.5 μm and the above titanium oxide
powder having a cumulative mass 50% diameter (d50) of 0.6 μm were
mixed in such a manner that the TiO2 content and the Fe2O3
content in the alumina sintered compact to be formed could be as in Table
7, thereby preparing various mixtures.

[0091] 300 g of an aqueous solution containing 5% by mass of polyvinyl
alcohol and 600 g of pure water were added to each mixture, ground and
mixed in a ball mill (processing time; 4 hours), thereby preparing
various types of homogeneous slurries each having a mixture concentration
of about 25% by mass.

[0092] Next, each slurry was dried at 120° C. for 24 hours, and
then ground in a mortar to give a ground powder having a cumulative mass
50% diameter (d50) of at most 300 μm. Each ground powder was
molded in a mold under a pressure of 100 MPa, and then further processed
for hydrostatic pressurization under a pressure of 150 MPa to give
various types of molded compacts.

[0093] Next, each molded compact was fired in an electric furnace (air
atmosphere) for 4 hours so as to have a relative density of at least 95%,
thereby giving various alumina sintered compacts. These were tested
(evaluated) as above. The results are shown in Table 7 below.

[0094] It has been confirmed through X-ray diffractiometry that, in the
alumina sintered compacts of Examples 1 to 7 and Comparative Examples 3
to 9, the metal oxide crystal phase containing Ti, Fe and Al and existing
in the grain boundary of the main crystal phase composed of a corundum
crystal is a crystal phase comprising FeTiAlO5.